Abstract

Background

Roo is the most abundant retrotransposon in the fruit fly Drosophila melanogaster. Its evolutionary origins and dynamics are thus of special interest for understanding
the evolutionary history of Drosophila genome organization. We here study the phylogenetic distribution and evolution of
roo, and its highly diverged relative rooA in 12 completely sequenced genomes of the genus Drosophila.

Results

We identify a total of 164 roo copies, 57 of which were previously unidentified copies that occur in 9 of the 12
genomes. Additionally we find 66 rooA copies in four genomes and remnants of this element in two additional genomes. We
further increased the number of elements by searching for individual roo/rooA sequence domains. Most of our roo and rooA elements have been recently inserted. Most elements within a genome are highly similar.
A comparison of the phylogenetic tree of our roo and rooA elements shows that the split between roo and rooA took place early in Drosophila evolution. Furthermore there is one incongruency between the species tree and the
phylogenetic tree of the roo element. This incongruency regards the placement of elements from D. mojavensis, which are more closely related to D. melanogaster than elements from D. willistoni.

Conclusion

Within genomes, the evolutionary dynamics of roo and rooA range from recent transpositional activity to slow decay and extinction. Among genomes,
the balance of phylogenetic evidence, sequence divergence distribution, and the occurrence
of solo-LTR elements suggests an origin of roo/rooA within the Drosophila clade. We discuss the possibility of a horizontal gene transfer of roo within this clade.